1. Field of the Invention
The present invention relates, in general, to methods for constructing a field oxide film which sections the substrate of semiconductor device into an active region and an device insulating field and, more particularly, to an improvement in securing more large active region along with the methods.
2. Description of the Prior Art
In the processes for manufacturing a semiconductor device, the construction of a field oxide film which divides the substrate into an active region and a field region is believed to be one of the most important processes to achieve a high degree of integration. In the construction, it is required to minimize the size of so-called "Bird's beak", to secure more large active region.
For better understanding of the background of the present invention, the description of a conventional method for constructing a field oxide film in a semiconductor device along with its problems is given next, with reference to FIGS. 1A through 1D showing the use of L-shaped spacer.
First, as illustrated in FIG. 1A, over a silicon substrate 11, there is formed a pad oxide film 12 on which a nitride film 13 and a first chemical vapor deposition (hereinafter referred to as "CVD") oxide film 14 are subsequently formed, in due order, and then, a selective etch process is undertaken to remove a predetermined portion of the first CVD oxide film 14 and nitride film 13, so as to form a field region. This figure further illustrates that a second nitride film 15 is formed on the resulting structure, followed by the formation of a second CVD oxide film 17 over the second nitride film 15. From these two second films, spacers are to be formed later.
Next, not until the pad oxide film 17 is exposed are the second CVD oxide film 17 and the second nitride film 15 subjected to etching, so that a pair of spacers are formed, as illustrated in FIG. 1B.
Thereafter, wet etching is applied to the spacers, to remove fragments of the second CVD oxide film left on the arms of the spacers, as illustrated in FIG. 1C. As a result, a pair of L-shaped spacers 15' of the second nitride film are formed.
Finally, a field oxide film 16 is formed by, for example, oxidation, as illustrated in FIG. 1D.
As inferred from the illustrated conventional method, the L-shaped spacer 15 shown in FIG. 1C dominates the subsequent growth of the field oxide film. In detail, the arm length of the L-shaped spacer 15' which is designated by reference letter l in FIG. 1C determines the area in which the silicon substrate 11 is oxidized when the field oxide film is formed. In the meanwhile, the thickness of the arm of the L-shaped spacer 15' which is designated by reference letter t dominates the shape of the surface s of the field oxide film formed. For example, if the thickness t is too large, Bird's beak comes to be shortened whereas the surface gets impressions pressed by the arms of the L-shaped spacers 15.
Accordingly, the conventional method for the construction of field oxide film is problematic in its processing steps and in controlling the thickness and the length. For example, controlling the thickness and length of the spacer always requires a precise control of the thicknesses of the oxide film and the nitride film both of which have been formed before the formation of the spacer.